514 CHEMICAL ENGINEERING
usually stored in bins, from which it is weighed out in the proper proportions, together with the ground-clay mixture. The materials are then mixed together in the dry state, avoiding further grinding, for which purpose a kneading machine may be employed. As soon as the dry mixing is completed water is added and the tempering continued until the soft plastic state has been attained and the whole mass is of uniform consistency throughout. The material is then allowed to cure or age for some time in the clay cellar. The crucibles are molded by means of the potters' jolly in plaster or metal molds lined with cloth. As regards to drying, the usual pottery practice is followed. The application of the modern humidity-drying methods, at least for the larger crucibles, should offer an opportunity for improvement, since uniformity of shrinkage is not always secured under tie present conditions.
The firing of the crucibles is carried to a temperature but little above the temperature of complete dehydration of the clay, usually between 650 to 700°C. Even in muffle kilns some loss of graphite occurs on the surface of the crucibles through oxidation, as is evidenced by the white coating of clay.
It is quite apparent to the casual observer that the treatment to which graphite crucibles are subjected is often extraordinarily severe. In the first place it is doubtful whether the function of drying out the crucibles is properly understood. Graphite crucibles are not fired to a high initial temperature and hence retain a porous structure. In common with all other clay products not burned to vitrification the crucibles are hygroscopic; that is, they greedily absorb moisture from the atmosphere, which is more difficult to expel than we generally realize.
Low-fired clays differ widely as to their hygroscopic capacity and the temperature at which they release the moisture thus absorbed. At the same time, the expulsion of this moisture requires considerable time. Therefore, it is not sufficient simply to keep the crucibles in a warm place, but they should be finally maintained for at least several days at a higher temperature, say 150°C., before being placed in the furnace. In Europe the crucibles on being taken from this kiln are sometimes covered with a waterproof coat, such as tar or pitch dissolved in turpentine.
The preheating of the crucible just before putting into the heat for first time, likewise, is often too abrupt, and hence the temperature change to which they are subjected too violent. The ideal method of handling the fresh crucibles would be to place them in a special furnace kept heated by the waste gases of the melting furnace, where the temperature could be brought up uniformly to somewhat below red heat.
The conditions of heating are of great importance. It is evident that the intense heat of an oil burner will bring a large crucible far closer to the softening point than when it is exposed to the fire of a coke furnace. The larger the crucible and the higher the level of the liquid metal, the lower in temperature must be the failing point of the crucible, since at furnace temperatures even comparatively small loads tend to bring about deformation. Add to these factors the effects of unequal heating and rough handling, and it can readily be seen why the life of crucibles varies so widely.
Silicon Carbide Refractories.—The use of silicon carbide, SiC, as a refractory is becoming more and more extensive. The material may be shaped without the use of a binder or with one such as clay or tar. In the former case cementation is brought about by the interlocking of the crystals in refiring in the electric furnace. In the second the strength must necessarily depend upon that of the bonding material and the refractoriness is limited by the latter, as well. Silicon carbide is being used quite largely now in crucibles, in which it displaces graphite.